1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor for detecting the concentration of a particular gas contained in, for example, combustion gas or exhaust gas of a combustor or an internal combustion engine.
2. Description of Related Art
Conventionally, various gas sensors are used for detecting the concentration of a particular component (oxygen, etc.) in exhaust gas of an internal combustion engine.
Such a gas sensor has a gas sensor element incorporated therein. The gas sensor element includes a ceramic plate-like element which encompasses a detecting section having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body.
One end portion (forward end portion) of the gas sensor element at which the detecting section of the plate-like element is located is exposed to exhaust gas. Accordingly, water, such as water droplets contained in exhaust gas and condensed water formed in an exhaust pipe, and poisoning substances, such as silicon and phosphorus, contained in exhaust gas may adhere to the plate-like element. Particularly, adhesion of water to the plate-like element may cause the occurrence of damage, such as cracking, to the plate-like element.
Thus, in order to capture poisoning substances and to prevent direct contact of water with the plate-like element, a forward end portion of the gas sensor element is covered with a porous protection layer formed of ceramic (refer to Patent Document 1).
Also, as shown in FIG. 12, the following technique has been developed: a porous protection layer P3 which covers a detecting section P2 located at a forward end portion of a gas sensor element P1 has a two-layer structure composed of a lower layer P4 and an upper layer P5 (which covers the entire surface of the lower layer P4).
One of methods of forming the porous protection layer P3 having the two-layer structure utilizes the dipping process described in Patent Document 1; specifically, a forward end portion of the gas sensor element P1 is dipped in a material slurry a plurality of times to form the lower layer P4 and the upper layer P5.